The present invention relates to secondary ion mass spectrometers, and more particularly to a secondary ion mass spectrometer suited to quantitatively determine a plurality of elements which are widely different from each other in concentration value or an element whose content varies in a wide range.
FIG. 4 is a block diagram showing the fundamental construction of a secondary ion mass spectrometer. Referring to FIG. 4, a primary ion emitting unit 1 produces and accelerates a primary ion 2, and focuses the primary ion beam thus obtained to scan a specimen 3 with the primary ion beam. Secondary ions 4 are sputtered from the specimen 3 bombarded with the primary ion beam. The secondary ions 4 thus obtained are separated according to mass-to-charge ratios by a mass spectrometer 5, and only a secondary ion 6 with a desired mass can pass through a detection slit 7 to reach a detection system 8, in which data indicative of the amount of the secondary ion 6 is stored in a memory, and the stored data is processed to be displayed by a display device.
A DC amplification method shown in FIG. 2 and a pulse measuring method shown in FIG. 3 have hitherto been used in secondary ion detection systems. In the DC amplification method, as shown in FIG. 2, a secondary electron multiplier 10 receives the secondary ion 6 with a desired mass through the detection slit 7, to convert the secondary ion 6 into an electron and to subject the electron to electron multiplication. A current 11 delivered from the secondary electron multiplier 10 is detected by a DC amplifier 18. When the resistance value of a feedback resistor 19 is fixed, the amount of secondary ion can be varied by four or five orders of magnitude within the dynamic range of the detection system. By changing th resistance value of the feedback resistor 19, the dynamic range of the detection system can be further enlarged. The resistance value of the feedback resistor 19 can be changed in a range from 10.sup.5 .OMEGA. to 10.sup.11 .OMEGA.. Accordingly, the detection system is able to have a wide dynamic range, within which the amount of secondary ion varies by ten orders of magnitude. However, in a case where the concentration distribution of an analytical element in the direction of thickness of a specimen is determined, as described in a U.S. Pat. No. 3,811,108 and a Japanese patent application Post-Examination Publication No. 53 -2,599, a detector is provided with electrical gate means, to remove secondary ions emitted from the side wall of a crater due to sputtering, thereby detecting only secondary ions emitted from the bottom of the crater. In this case, the DC amplifier 18 is required to respond faithfully to variations in the amount of emitted secondary ion among points scanned with a primary ion. Accordingly, even if the resistance value of the feedback resistor 19 is changed as mentioned above, the detection system will have a dynamic range, within which the amount of secondary ion varies only by seven to eight orders of magnitude.
While, in the pulse measuring method, the secondary ion with a desired mass is treated as a pulse. Referring to FIG. 3, one secondary ion impinges on a secondary electron multiplier 10, to be subjected to electron multiplication, and the pulse current 11 thus obtained is amplified by a high-speed pulse amplifier 12. When the height of the pulse current 11 exceeds a predetermined level set in a discriminator 13, a pulse counter circuit 14 counts up this pulse. As mentioned above, in this method, secondary ions are counted up in a time-divisional manner, and hence the maximum number of secondary ions detectable in a unit time (that is, the maximum detectable secondary ion current) is determined by the response time of each of the high-speed pulse amplifier 12, discriminator 13 and pulse counter circuit 14. Accordingly, this detection system will have a dynamic range, within which the amount of secondary ion varies by six to eight orders of magnitude.
As mentioned above, a detection system using one of the DC amplification method and the pulse measuring method will have a dynamic range, within which the amount of secondary ion varies by eight orders of magnitude at most.
Hence, when a specimen is analyzed by a secondary ion mass spectrometer, it is necessary to satisfy the following requirements. (1) The concentration of an analytical element is greater than that of another analytical element by less than eight orders of magnitude. (2) The concentration of an analytical element varies in a specimen by less than eight orders of magnitude.
As to the first requirement (1), it is to be noted that when a low-concentration impurity in a matrix is quantitatively determined together with a matrix element, the concentration of the matrix element is usually greater than that of the impurity by eight or more orders of magnitude, and thus the detection value of the matrix element will reach a saturation value. In order to solve this problem, one of a multiply-charged ion and a complex ion which are smaller in number than a matrix ion, is detected instead of the matrix ion. However, the switchover of the matrix ion to the multiply-charged ion or complex ion is cumbersome.
As to the second requirement (2), it is to be noted that in a case where the concentration distribution of an impurity in a multi-layered film, in which the impurity can be diffused from one of adjacent layers into the other layer, is detected, or in a case where a specimen, in which the concentration of an impurity is changed from a large value to a considerably small value, is analyzed, the concentration of the impurity will vary by eight orders of magnitude or more, and thus it is impossible to determine the concentration distribution of impurity accurately.
As is evident from the above, a conventional secondary ion mass spectrometer has paid no attention to the enlargement of the dynamic range of a detection system, and hence cannot determine a high-concentration element and a considerably-low-concentration element quantitatively at the same time nor determine the concentration distribution of an analytical element whose concentration varies in a wide range from a large value to a considerably small value.